1. Field in the Invention
The present invention relates to stator windings for use in electric motors and more particularly, to reducing the risk of motor failure caused by damage to the insulation of the stator windings.
2. Relevant Art
During the manufacture of electric motors, lengths of wire, called "conductors", are wound around the stator structure. This winding process usually causes the windings and internal motor leads to be bent into relatively narrow radius curves, which can often times cause the insulation on the conductors to be damaged. This damage is in the form of cracks when the conductors are coated with a hard coating, such as varnish, to immobilize the conductors. When an electric motor is used in a high temperature and/or in a corrosive environment, these cracks in the insulation can cause premature failure of the motor. For example, when an electric motor is used in an electric submergible pump (ESP) that is installed in a wellbore to recover fluids, premature failure of the motor requires the operator to retrieve or pull the failed motor. Such retrievable operations are relatively expensive and time consuming, and the operator suffers lost revenue while the production of the fluids from the well is halted. There is a need for improved stator structure and a method of making same which will reduce the likelihood of such damage to the insulation of the conductors, and thus eliminate or minimize the risk of resulting motor failure.
A commonly used insulation system for ESP electric motors includes insulating sleeves and insulating laminations at opposite ends of the stator, insulating slot liner tubing for the stator windings, multiple layers of insulation on the magnet wire of the windings, insulating tape on the end turns of the windings, insulating sleeves on the connections to the end turns, and an insulating immobilization agent such as varnish or epoxy, that impregnates the stator structure. For example, U.S. Pat. No. 4,275,319, which is commonly assigned hereto, discloses an improved hydrolytically stable varnish for use in an ESP's electric motor to extend the life of such motor. The varnish serves as a moisture barrier to protect the magnet wire insulation from hydrolytic attack. The varnish also serves as a secondary electrical insulation for the electrical conductors of the stator, supports the weight of the stators windings, and encapsulates the end turns of the stator windings so that they are substantially immobilized. In certain instances, a varnish-less motor can be utilized with an ESP when the winding are covered with a particular type of insulation material, usually polyetheretherketone (PEEK) or polyetherketone (PEK), as disclosed in U.S. Pat. No. 4,833,354, which is commonly assigned hereto.
The problem of damage to the conductors' insulation of an ESP motor occurs during the manufacture when the windings are bent into relatively short radius curves, and the motor leads (that extend from each stage of the motor) are bent to make the electrical interconnection means. Once the stator structure has been covered with varnish or epoxy, any further bending of the winding's end turns and the motor leads most likely cause cracks in the varnish. It has been found by the inventor hereof that the typical immobilization agent, such as the varnish or epoxy, adheres or bonds relatively tightly to the exterior surface of the insulation and especially to PEEK or PEK. If stress (ie. bending) and/or tensile forces are applied to the conductors, of sufficient magnitude to cause a crack in the varnish or epoxy, this crack propagates into the insulation because the underlying insulation is subjected to tensile forces from the varnish that tears the insulation to expose the bare magnet wire. These cracks and tears have led to premature electrical failure of electric motors which have necessitated costly retrieval, replacement and repair operations. As described earlier, there is a need for an improved stator winding and a method of making same which will reduce the likelihood of such damage to the conductors' insulation and resulting motor failure.